A method is conventionally known of measuring the oxygen activity of the slag in molten iron or molten steel by winding in a close contact state a platinum contact electrode around the outer surface of a solid electrolyte in the form of a tube having one end closed or by coating the outer surface thereof with a platinum contact electrode to prepare an oxygen concentration cell, embedding the oxygen concentration cell in the end of the probe, and disposing a probe directly in a molten slag layer which is existent in the upper layer of the molten steel in a converter or a ladle during operation.
Since the surface of the slag flows violently, however, it is difficult to exactly locate the oxygen concentration cell, which serves as a detecting portion, in the slag, so that the times at which measurement succeeds is low. Since the detecting portion is exposed to a high temperature for a long time, a firm heat-resistant material is necessary, therefore the probe becomes thick and heavy, and thus difficult to handle. In addition, it is necessary to bring the platinum into contact with the surface of the solid electrolyte in order to produce the oxygen concentration cell in the above-described form. Since the solid electrolyte is densified by the heat of the slag during operations, it is difficult to constantly maintain the contact state and thus a defective contact between the solid electrolyte and the contact electrode is produced, which results in the production of an error in measurement. Furthermore, although it is necessary to stop the progress of the steelmaking process for the measurement, it is actually impossible to stop the steelmaking process for a long time. It is therefore impossible to use a probe-type oxygen measuring apparatus in an industrial furnace.
Accordingly, it is an object of the present invention to solve these problems in the prior art and to provide a method of measuring the oxygen activity of slag which is capable of measuring the oxygen activity of a slag with high accuracy and to provide an oxygen activity measuring apparatus which is used for the method and which is very easy to handle.